Date and time:  January 21st  2019

Speaker: Rupert Coy

Abstract: In the absence of direct evidence of new physics, any UV theory can be reduced to its specific set of low-energy effective operators. As a case study, I derive the EFT for the seesaw extension of the Standard Model, with sterile neutrinos of mass M > m_W. All  Wilson coefficients generated at 1-loop are systematically computed. Hence, it becomes straightforward to (i) identify the seesaw parameters compatible with the smallness of neutrino masses; (ii) compute precision lepton observables, which may be sensitive to scales as large as M ? 10^3 TeV; and (iii) establish sharp correlations among those observables. I find that the flavour-conserving Wilson coefficients set an upper bound on the flavour-violating ones. The low-energy limits on \mu -> e and \tau -> e,\mu transitions suppress flavour violation in Z and Higgs decays, as well as EDMs, far beyond the experimental reach.The precision measurements of G_F, m_W, and the invisible Z width set more stringent bounds than present and future limits on \tau -> e,\mu transitions. I will also present a spurion analysis to compare the seesaw with different models, thus assessing the discriminating potential of the effective approach.